The present invention relates to an austenitic stainless steel alloy with high contents of Cr, Mo, Mn, N and Ni for applications within areas where a combination of good corrosion resistance are required, for example against normally occurring substances under oil and gas extraction, as well as good mechanical properties, such as high strength and fatigue-resistance. It should be possible to use the steel alloy for example within the oil and gas industry, in flue gas cleaning, seawater applications and in refineries.
Austenitic stainless steels are steel alloys with a single-phase crystal structure, which is characterized by a face-centered cubic-lattice structure. Modern stainless steels are primarily used in applications within different processing industries, where mainly requirements regarding to corrosion resistance are of vital importance for the selection of the steel to be used. A characteristic of the stainless austenitic steels is that they all have their maximum temperature in the intended application areas. In order to increase applicability in difficult environments, alternatively at higher temperatures, higher contents of alloying elements such as Ni, Cr, Mo and N been added. Primarily the materials have been used in annealed condition, where yield point limits of 220-450 MPa have been usual. Examples of high alloyed stainless austenitic steels are UNS S31254, UNS N08367, UNS N08926 and UNS S32654. Even other elements, such as Mn, Cu, Si and W, occur either such as impurities or in order to give the steels special properties.
The alloying levels in those austenitic steels are limited upwards by the structural stability. The austenitic stainless steels are sensitive for precipitation of intermetallic phases at higher alloying contents in the temperature range 650-1000° C. Precipitation of intermetallic phase will be favored by increasing contents of Cr and Mo, but can be suppressed by alloying with N and Ni. The Ni-content is mainly limited by the cost aspect and because it strongly decreases the solubility of N in the Smelt. The content of N is consequently limited by the solubility in the smelt and also in solid phase where precipitation of Cr-nitrides can occur.
In order to increase the solubility of N in the smelt, the content of Mn and Cr can be increased as well as the content of Ni can be reduced. However, Mo has been considered to cause an increased risk of precipitation of intermetallic phase and for this reason it has been considered being necessary to limit this content. Higher contents of alloying elements have not only been limited by considerations regarding the structural stability. Even the hot ductility during the production of steel billets has been a problem for subsequent working.
An interesting application of stainless steel is in plants for the extraction of oil/gas or geothermal heat. The application puts high demands on the material due to the very aggressive substances hydrogen sulfide and chlorides, in different conditions dissolved in the produced liquids/gases, such as oil/water or mixtures thereof at very high temperatures and pressure. Stainless steels are used here in large degree both as production tube and so-called wirelines/slicklines down in the sources. The degree of resistance against chloride induced corrosion of the materials, H2S-induced corrosion or combinations thereof can be limiting for their use. In other cases, the use is limited in larger degree by the fatigue-resistance due to repeated use of the alloy as wireline/slickline and from the bending of the wire over a so-called pulley wheel. Further, the possibilities to use the material within this sector are limited by the permitted failure load of wireline/slickline-wires. Today the failure load will be maximized by use of cold-formed material. The degree of cold deformation will usually be optimized with regard to the ductility. Corresponding requirement profiles can be needed for strip- and wire-springs, where high requirements on strength, fatigue- and corrosion properties occur.
Usually occurring materials within this sector for use in corrosive environments are UNS S31603, duplex steels, such as UNS S31803, which contains 22% Cr, UNS S32750, which contains 25% Cr, high alloyed stainless steels, such as UNS N08367, UNS S31254 and UNS N08028. For more aggressive environments, exclusive materials such as high alloyed Ni-alloys with high contents of Cr and Mo and alternatively Co-based materials are used for certain applications. In all cases the use is limited upwards by reasons of corrosion and stress.
When considering a steel for use in these environments it is well-known that Cr and Ni increase the resistance to H2S-environments, while Cr, Mo and N are favorable in chloride environments according to the well-known relationship PRE=% Cr+3.3% Mo+16% N. An optimization of an alloy has until now led to the contents of Mo and N being maximized in order to obtain the highest possible PRE-value in that way. Thus, in many of the presently existing modern steels the resistance to a combination of H2S- and Cl-corrosion has not been given priority, but only in a limited extent been taken into account. Further, oil extraction today is being done to an increasing extent from sources becoming deeper and deeper. At the same time the pressure and temperature increase (so called High-pressure, High temperature Fields). Increased depth leads of course to an increased dead weight during use of free hanging materials, whether these concerns so called wirelines or pipe tracks. Increasing pressure and temperature leads to the corrosion conditions aggravating so that the requirements on the existing steel increase. For wirelines, there are also requirements to increase the yield point in tension since there occurs plasticity on the surface of the existing materials at the presently used sizes of pulley wheels. Tension stresses up to 2000 MPa exist in the surface layer, which is considered strongly contributing to the short lifetime, that is obtained for wireline-alloys.
In the light of the above background, it is easy to identify a requirement for a new alloy, which combines both the resistance to chloride-induced corrosion and resistance to H2S-corrosion for applications particularly in the oil and gas industry, but also within other application areas. Further, there exist demands on significantly higher strength than today's technique achieves at a given range of cold-deformation. As strength is wanted which leading to that normally occurring dimensions of wire do not plastify on the surface or allowing the use of smaller dimensions is desired.
In U.S. Pat. No. 5,480,609, an austenitic alloy is described, which according to claim 1 contains iron and 20-30% chromium, 25-32% nickel, 6-7% molybdenum, 0.35-0.8% nitrogen, 0.5-5.4% manganese, highest 0.06% carbon, highest 1% silicon, all counted on the weight, and which exhibits a PRE-number of at least 50. Optional components are copper (0.5-3%), niobium (0.001-0.3%), vanadium (0.001-0.3%), aluminum (0.001-0.1%) and boron (0.0001-0.003%). In the only practical example 25% chromium, 25.5% nickel, 6.5% molybdenum, 0.45% nitrogen, 1.5% copper, 0.020% carbon, 0.25% silicon and 0.001% sulfur, balance iron and impurities were used. This steel exhibits good mechanical properties, but has not sufficiently good properties to fulfill the purposes according to the present invention.